The present invention relates to sorbents used for the analysis of organic contaminants, particularly to sol-gel derived sorbents and more particularly to copolymerized sol-gel derived sorbents with controlled pore sizes and surface areas used as air-sampling sorbents for the analysis of organic contaminants including organic explosives.
Until recently, the needs for improved solid sorbent sampling media have been largely unrecognized. Dramatic improvements in sensor and analytical instrument sensitivity have relegated interest in improved sampling materials to secondary status. However, existing regulatory drivers and/or remediation/containment monitoring requirements are forcing the environmental analytical chemist to detect analytes at lower levels where instrumental sensitivity improvements are more difficult to achieve. Sample concentration, prior to sensing or analytical detection, will be required to achieve the more sensitive detection limits.
Solid sorbents have been used for a number of years for sampling of environmental contaminants. The use of small, robust multisorbent traps already has found application in mainstream analytical methodologies and exhibited the potential for substantial cost savings. For example, described in the Compendium of Methods for the Determination of Toxic Organic Compounds in Ambient Air are U.S. Environmental Protection Agency methods for monitoring volatile and semivolatile organic compounds in air samples, several of which employ sorbent sampling techniques. In fact, each of the first two methods calls for a different sorbent sampling technique, followed by thermal desorption to a capillary gas chromatograph column for analysis. In Method TO-1, compounds are trapped on TENAX(trademark) (poly(2,6-diphenyl)-phenylene oxide, Enka Research Institute Arnhem), a porous polymer adsorbent, transferred to a cold trap, and desorbed to the column. In Method TO-2, compounds are trapped on a carbon molecular sieve adsorbent, transferred to a cold trap, and desorbed to the column.
Various materials have been developed over the years for use as sorbents for capturing and releasing organic analytes. Activated charcoal was one of the first widely used solid sorbent materials. However, its properties are such that, for many analytes, it does not release the sorbate under a solvent wash, and can often promote chemical transformation of the sorbed analytes. Today, activated charcoal is typically used for industrial hygiene applications, where the stability of the analyte in question has been confirmed, and quantities of the target analyte are not small. Porous polymers, such as the CHROMOSORB 106(trademark) (styrene/divinylbenzene copolymer, Manville Corporation) and TENAX-GC(trademark), increased in popularity because they could be thermally desorbed and reused. However, TENAX-GC(trademark), the most widely used of these sorbents, suffers of poor retention of polar molecules and more volatile species. More importantly, TENAX(trademark) can decompose, slightly, when it is heated, resulting in a number of artifacts being observed in supposed blank samples.
In the late 1980""s,nonspecific carbon-based sorbents, based on sintered carbon blacks and carbon molecular sieves, became commercially available. These materials have excellent thermal stability, and because of their nonspecific adsorption characteristics, are useful for collection of several types of organic volatiles or semivolatiles. Carbon sorbents are also known to have xe2x80x9cgoodxe2x80x9d thermal stability and are good sorbents for a wide range of organics such as hydrocarbons, chlorinated solvents, nitriles, ketones, etc. However, the carbon-based solid sorbents utilized in currently used systems suffer from some important limitations. The volatility range of analytes which any one sorbent can collect is relatively narrow, necessitating the use of traps filled with the multiple sorbents described above in a xe2x80x9cparfaitxe2x80x9d configuration. That is, the sorberts are packed sequentially in a bed, with increasingly retentive sorbents downstream. The traps can be thermally desorbed, during which time the desorption flow is the reverse of that used for sampling. While these so-called xe2x80x9cmulti-sorbent trapsxe2x80x9d are extremely useful in a variety of active air sampling applications, the multilayer configuration of the traps precludes their use for passive sampling. Also, if the thermal pulse during the desorption phase of the analysis is not closely synchronized with the desorption flow, sorbates can be pushed off one sorbent bed and be irreversibly sorbed on a more retentive sorbent bed.
The silicate sol-gel technique is a process whereby a tetraalkylorthosilicate such as tetramethylorthosilicate (TMOS), is hydrolyzed under acid or base catalysis to first produce a sol which subsequently gels and proceeds to form a xerogel. The sol is a colloidal suspension of small (1 nm to 1000 nm diameter) particles of polymerized silicate. The suspension is thermodynamically stable and upon further reaction proceeds to produce a gel. The gel is a solid structure formed from the reacting silicate and containing within it a continuous liquid phase. Further curing of the gel by removal of all trapped water leads to the formation of a xerogel. The low processing temperatures used to form these glasses allow for construction of inorganic materials with entrapped organic guests. The sol-gel process is typically used to produce highly hydroxylated materials.
Materials for use as organic analyte sorbents must contain covalently bonded nonpolar moieties to promote favorable analyte-substrate interactions. Several methods have been developed that allow for the production of composite inorganic-organic materials. Alkyltrialkoxysilanes, such as methyltrimethoxysilane (MTMS), can be polymerized to form xerogels wherein an alkyl substituent is attached directly to silicon (Si) atoms in the backbone of the polymer network. These materials have organic moieties throughout and are substantially different from surface derivatized materials. Another approach to preparing composite inorganic-organic materials involves derivatization of the surface of inorganic oxides with a variety of organic reagents. This approach has been widely employed to modify the surface of chromatographic silica gels, for example. A variety of technologies for preparation of siloxane, [SiO2]xe2x80x94Oxe2x80x94Sixe2x80x94R, bonded materials exists. These methods often produce materials having varied thermal and hydrolytic instabilities. One reason for their instability is that methods based on derivatizing surface hydroxyl groups of silica-based gels are not able to cap all hydroxyl groups as a result of the bulky size of the derivatizing agents. Underivatized and exposed surface hydroxyl groups contribute to the thermal and hydrolytic instability of many supports.
An approach that has been used to circumvent this problem involves the use of trifunctional silanes such as RSiCl3 and RSi(ORxe2x80x2)3 as surface modifying reagents. It has been reported in Analytical Chemistry, v. 65 (1993), pp. 822-826, that trichlorosilanes can be induced to undergo attachment with simultaneous lateral polymerization to form a siloxane coating, when the reaction is conducted on a silica gel that contains a monolayer of water on its surface. Although not studied in detail, the polymeric bonded phases were reported to have substantially improved hydrolytic stabilities even at pH 1.8 and 10.0. This research was aimed at making high carbon density surfaces with mixed C3 and C18 coatings for chromatography purposes. However, the procedure is general for other xe2x80x9cRxe2x80x9d groups.
Another intriguing approach stems from a recent report in Journal of American Chemical Society, vol. 117, pp. 2112-2113 on the reactions of ethoxysilanes with silica gels. Highly dehydroxylated silica gels were prepared by heating at 600xc2x0 C. Reaction of this material with a monoethoxysilane reagent gave a highly derivatized surface, as determined by solid-state NMR, that quantitatively retained the ethoxy groups. This finding was interpreted as involving reaction with surface siloxane groups, generated by the heat treatment, as shown below.
(CH3)3SiOCH2CH3+[SiO2]O[SiO2]xe2x86x92(CH3)3SiO[SiO2]+CH3CH2O[SiO2]
The key feature is that as the siloxane bridges are opened, each trimethylsilyl group added has an adjacent ethoxy group.
Methyl-capped inorganic oxides can be produced by treating a hydroxylated material with methanol. Silica gels produced by this technique are thermally stable up to 600xc2x0 C. to 650xc2x0 C. before methyl moieties are lost as described by C. Morterra et al., J. Phys. Chem., v. 73 (1969), pp. 321-326. The use of alkyl-capped inorganic oxides in aqueous systems is severely limited by rapid hydrolysis to give methanol and the hydroxylated inorganic oxide.
Explosives vapor sampling is an important part of operational protocols for bomb scene investigations. Vapors containing explosives can be generated by gently heating debris in a closed container and collecting the headspace onto an adsorbent. Volatile explosives can also be collected on adsorbent tubes from the rubble and debris at a bomb scene. Sorbents that have reportedly been used to collect explosives vapors include activated charcoal, Tenax(trademark), Tenax-GC(trademark), Porapak Q(trademark), Thermosorb/N(trademark), Amberlite XAD-7(trademark), and silica. The most commonly reported method for removing the explosives from the sorbent bed is by washing with organic solvents. Solvents that have been used to remove explosives from various adsorbent beds include methylene chloride, acetone, 2-propanol, ethyl acetate, methyl tertiary-butyl ether, methylene chloride/methanol mixture, pentane, and pentane/methyl tertiary-butyl ether mixture.
Thermal desorption of organic explosives from adsorbent traps containing Tenax-GC(trademark), Tenax(trademark), and from a concentric tube adsorption device has also been reported. Hobbs and Conde in xe2x80x9cComparison of different techniques for the headspace analysis of explosivesxe2x80x9d, Proc. 3rd Int. Symp. on Analysis and Detection of Explosives, have shown that thermal desorption from Tenax(trademark) was unsatisfactory using a commercial desorption unit that focused the analytes on a Tenax(trademark) bed. A second thermal desorption method, the xe2x80x9cTenax(trademark) needlexe2x80x9d method, was also tested. The Tenax(trademark) needle method involved passing the explosives through a standard injection port following thermal desorption from a Tenax(trademark) sorption tube. This method allowed chromatographic analysis of the headspace above commercial explosives. Detection limits were not reported for the Tenax(trademark) needle method. The use of solid-phase microextraction has also been reported for the collection and analysis of organic explosives by headspace sampling, and for sampling explosives and their metabolites in seawater.
Accordingly, it is an object of the present invention to provide sol-gel derived sorbents and copolymerized sol-gel derived sorbents that are capable of efficiently binding organic materials.
It is another object of the present invention to provide a method for producing copolymerized sol-gel derived sorbents having controlled pore size and surface areas, thermal stability and purity of the material that capable of efficiently binding organic materials.
It is yet another object of the present invention to provide a method for using copolymerized sol-gel derived sorbents for the analysis of organic contaminants including the analysis of organic explosives.
It is a further object of the present invention to provide sol-gel derived sorbents including copolymerized sol-gel derived sorbents that are thermally stable.
It is still yet another object of the present invention to provide sol-gel derived sorbents including copolymerized sol-gel derived sorbents that are highly pure materials.
Further and other objects of the present invention will become apparent from the description contained herein.
In accordance with one aspect of the present invention, the foregoing and other objects are achieved by a method for producing copolymerized sol-gel derived sorbent particles for the production of copolymerized sol-gel derived sorbent material with controlled pore size and surface area used as air-sampling sorbents for the analysis of organic contaminants and for the analysis of organic explosives. The copolymerized sol-gel derived sorbent particles have thermal stability and high purity. The method for producing copolymerized sol-gel derived sorbent particles comprise adding a sufficient amount of a basic solution to an aqueous metal alkoxide mixture which comprises at least two different metal alkoxides, to hydrolyze the metal alkoxides wherein the mixture has a pHxe2x89xa68. Then, allowing the mixture to react at room temperature for a desired, precalculated period of time for the mixture to undergo a desired increase in viscosity so to obtain a desired pore size and surface area. Then, adding the copolymerized mixture to a nonmiscible, nonpolar solvent that has been heated to a sufficient temperature wherein the copolymerized mixture forms a solid upon addition to the heated nonpolar solvent. Then, recovering the solid from the copolymerized-nonpolar solvent mixture.
In accordance with another aspect of the present invention, other objects are achieved by a method for using copolymerized sol-gel derived sorbent particles wherein the method comprises providing an active sampling trap containing copolymerized sol-gel derived sorbent particles wherein the copolymerized sol-gel derived sorbent particles comprise copolymerized metal alkoxides forming nonpolar sorbent particles. Then, exposing the active sampling trap to a sample containing organic analytes at a rate sufficient enough to permit the organic analytes to contact and adsorb onto the copolymerized sol-gel derived sorbent particles.
Still further objects are achieved by a method for using copolymerized sol-gel derived sorbent particles wherein the method comprises providing a passive sampling trap containing copolymerized sol-gel derived sorbent particles wherein the copolymerized sol-gel derived sorbent particles comprise copolymerized metal alkoxides forming nonpolar sorbent particles. Then, exposing the passive sampling trap to a sample containing organic analytes for a period of time sufficient enough to permit the organic analytes to contact and adsorb onto the copolymerized sol-gel derived sorbent particles.